LAB EXERCISE 9
SENSES: SIGHT- STRUCTURE AND FUNCTION OF THE
EYES.
WORK IN GROUPS OF 3 - 4
Objectives:
Describe the structure and function of the accessory visual structures.
- Describe the gross and microscopic structure of the eye and relate structure to function.
Outline the use of the ophthalmoscope and describe the features of the normal eye seen with it.
Explain the existence of the blind spot.
- Explain the mechanism of image formation on the retina.
- Trace the visual pathway to the optic cortex.
Define visual acuity, describe how it is measured, and explain the factors which determine it.
Describe and explain the three eye reflexes related to near vision.
Describe common errors of refraction.
Sense organs are specialized receptors that enable the body to detect changes in the environment. Sensory nerves associated with sensory receptors transmit this information through the peripheral nervous system to the central nervous system where it is perceived as sound, smell, sound or sight. The interpretation of conscious sensation is called perception. The sense organ studied in this exercise is the eye.
The eye houses the receptors which are stimulated by light and are able to convert this energy into electrical energy. This electrical energy, in the form of a nerve impulse, travels from the eye's sensory neurons along the optic nerve (cranial nerve III) to the occipital lobe of the cortex where it is interpreted as a visual image.
PART I: ANATOMY.
I. EXTERNAL FEATURES AND ASSOCIATED STRUCTURES
(Seeley p. 508-509)
EXERCISE A.
- equipment -
- figure 1
Study your partner's eye.
Identify the upper and lowereyelids. Note the eyelashes. Infection of the sebaceous glands associated with them is known as a sty. Locate the tarsalglands (Seeley p. 508). They are modified sebaceous glands that produce an oily secretion that prevents the eyelids from sticking together.
The lacrimal apparatus consists of the lacrimal gland, lacrimal canals, lacrimal sac and the nasolacrimal duct (Seeley p. 509). The lacrimal glands continually liberate a dilute salt solution: the tears. They keep the exposed surface of the eye moist and free of dust and microorganisms. Find the openings of the lacrimal ducts. Trace the path through which lacrimal fluid is drained from the eye. Note the sclera, cornea, iris and pupil. Make sure that you know the boundary between the sclera and the cornea (Seeley p. 511). Find the conjunctiva.
Ask your partner to move her eyeballs up and down, right to left and in a circular motion. The movements of each eyeball are controlled by six extrinsic eye muscles (Seeley p. 510). They are skeletal muscles and they originate from the bony orbit and insert into the outer surface of the eyeball.
II. STRUCTURE OF THE EYEBALL
(Seeley 511-514)
EXERCISE B.
- Equipment -
eye model
- dissection on display
Study the models, the dissection on display and your prelab exercise. Identify the following structures and their functions: sclera, cornea, conjunctiva, choroid, ciliary body (muscles and processes), suspensory ligament, iris, pupil, lens, anterior chamber, posterior chamber, aqueous humor (in anterior cavity), vitreous humor (in posterior cavity), retina, macula lutea, fovea centralis, optic disc (= blind spot), optic nerve.
During its passage, the light is bent three times: on entry into the cornea and on entering and leaving the lens. The aqueous and vitreous humors are of minimal importance in light refraction. The cornea is responsible for most of the light refraction in the eye, but since its thickness is constant, its refractory power is unchanging. On the other hand, the lens is highly elastic and its curvature can be actively changed to allow fine focusing of the image.
When the eye is at rest, it is focused for distant vision: the suspensory ligaments are under tension and stretch the lens, making it flatter. Smooth muscle fibers forming the ciliary muscle are imbedded within the ciliary body. When they contract, the suspensory ligaments are pulled forward (toward the cornea). This shortens the fibers of the ligaments and reduces the tension, thus permitting the lens to become thicker (i.e. more nearly round). This provides accommodation for near vision. This change in lens shape bends the light rays more sharply and permits light rays from near objects to be focused on the retina. Accommodation of the lens is an autonomic reflex.
Smooth muscle within the iris regulates the size of the pupil. Dilation of the pupil is caused by a contraction of the radial muscles of the iris. Pupils are constricted by circular muscles of the iris. The iris constricts in bright light and dilates when the available light is dim. It also constricts in accommodation for near vision to prevent the most divergent light rays from entering the eye: these rays would pass through the extreme edge of the lens, would not focus properly and would cause blurred vision.
Accommodation of the lenses and constriction of the pupils are two of the three reflexes that occur simultaneously when focusing for close vision. The third reflex occurring is the convergence of the eyeballs, its goal being to keep the object being viewed focused on the retinal fovea of each eye. The signal that induces this trio of reflex responses appears to be a blurring of the retinal image.
NAME & SECTION #:
PART II: VISUAL TESTS AND EXPERIMENT.
REMEMBER: DO NOT COPY WORD FOR WORD FROM YOUR TEXTBOOK
I. OPHTHALMOSCOPIC EXAMINATION OF THE EYE
(Seeley p. 513, 524)
The ophthalmoscope is an instrument used to examine the fundus, or eyeball interior, to determine visually the condition of the retina, optic disk and internal blood vessels. Certain pathological conditions such as diabetes, arteriosclerosis and degenerative changes of the optic nerve and retina can be detected by such an examination. The ophthalmoscope consists of a set of lenses mounted on a rotating disk (the lens selection disk), a light source regulated by a rheostat control, and a mirror that reflects the light so that the eye interior can be illuminated (figure 2). The lens selection disk is positioned in a small slit in the mirror, and the examiner views the eye interior through this slit, appropriately called the viewing window. The focal length of each lens is indicated in diopters preceded by a + sign if the lens is convex and by a - sign if the lens is concave. When the zero (0) is seen in the diopter window, there is no lens in position in the slit. The depth of focus for viewing the eye interior is changed by changing the lens.
The light is turned on by depressing the red rheostat lock button and then rotating the rheostat control in the clockwise direction. The aperture selection disk on the front of the instrument allows the nature of the light beam to be altered (generally a green light beam allows for clearest viewing of the blood vessels in the eye interior and is most comfortable for the subject).
Now that you are familiar with the ophthalmoscope, you are ready to conduct an eye examination.
EXERCISE C.
All students in the group must examine the fundus.
- Equipment -
- ophthalmoscope
1. Conduct the examination in a dimly lit or darkened room with the patient comfortably seated and gazing straight ahead. To examine the right eye, sit face-to-face with the patient and hold the instrument in your right hand. Use your right eye to view the eye interior. To view the left eye, use your left eye, and hold the instrument in your left hand. When the ophthalmoscope is correctly set, the fundus should appear as shown figure 15.13 p. 513 in Seeley.
2. Begin the examination with the 0 (no lens) in position. Hold the instrument so that the lens disk may be rotated with the index finger. Hold the ophthalmoscope about 6 inches from the patient's eye and direct the light into the pupil at a slight angle (about 25o: through the pupil edge rather than directly through its center). You will see a red circular area that is the illuminated eye interior.
3. Move in as close as possible to the subject's cornea as you continue to observe the area. Steady your instrument-holding hand on the patient's cheek if necessary. If both your eye and that of the patient are normal, the fundus can be viewed clearly without further adjustment of the ophthalmoscope. If the fundus cannot be focused, slowly rotate the lens disk counterclockwise until the fundus can be clearly seen. (Note: If a positive (convex) lens is required and your eyes are normal, the patient has hyperopia. If a negative (concave) lens is necessary to view the fundus and your eyes are normal, the patient is myopic). If you are unable to achieve a sharp focus or to see the optic disc, move medially or laterally and begin again.
4. Examine the optic disc for color and sharpness of outline. Observe the blood vessels radiating from near its center. Locate the macula, which is lateral to the optic disc. It is a darker area in which blood vessels are absent and the fovea appears to be a slightly lighter area in its center. The macula is most easily seen when the subject looks directly into the light of the ophthalmoscope
5. Draw the posterior wall of the retina as you see it through the ophthalmoscope (label the blood vessels, the optic disc, the macula and the fovea).
6. Define the following structures seen in the fundus. What are their particularities?:
- optic disc:
- macula lutea and fovea centralis:
II. THE BLIND SPOT DISTANCE
(Seeley p. 513)
EXERCISE D.
- Equipment -
- blank 3 x 5 card.
- black felt marker pen.
- a ruler
Table 1: Blind spot distance in cm.
STUDENT'S NAMES / LEFT EYE / RIGHT EYEMEAN
1. Draw a small (8 mm high) but welldefined black cross on the lefthand side of a blank 3 x 5 card. About 6 cm (2 1/2 inches) to the right draw a solid black circle 4 mm in diameter as shown in figure 3.
2. Close the left eye, and with the right eye, look steadily at the cross. Hold the card about 20 cm (8 inches) from the eye. Still looking steadily at the cross, move the card slowly towards the eye until the circle disappears. Record the distance at which this occurs in table 1.
What happens if you move the card still nearer to the eye: does the circle reappear or not? What is your explanation for the disappearance of the circle?
What is your explanation for what is happening when you move the card nearer to the eye?
3. Locate the distance at which the circle disappears from vision for the right eye (blind spot distance; see 2.), then close the right eye and look steadily at the circle with the left eye. What happens to the cross?
What is your explanation?
Record the blind spot distance for the left eye in table 1.
Still looking at the circle with the left eye, move the card nearer or farther away. Why does the cross reappear?
- CLOSE VISION.
(Seeley p. 515-516, 524-525)
Our eyes are best adapted for distance vision. Rays of light coming from an object far away approach the eye nearly parallel to each other and are focused precisely on the retina by the refractory apparatus of the eye (cornea and humors which are fixed and the lens which is, in this case, stretched). To look at distant objects, we only need to aim our eyeballs so that they are both fixated on the same spot. The far point of vision is that distance beyond which no change in lens shape is needed for focusing. For the normal or emmetropic eye, the far point is 6 meters (= 20 feet).
Light rays from objects less than 6 meters away diverge as they approach the eyes and come to a focal point farther from the lens. Thus, close vision demands that the eye makes active adjustments. To restore focus, three processes must be initiated simultaneously. The closest point on which we can focus clearly is callednear point of vision.
EXERCISE E.
- Equipment -
- piece of dark string or cord (about 2 meters long)
- masking tape
- clothes pin
- meter stick
1. Tape the string to the wall at eye level and place the clothespin on it so that it moves freely along the string. Ask your subject to hold the free end of the string taut to the tip of her nose and position the clothespin about half way along the string. Ask her to focus on the clothespin.
2. Move the clothespin along the string, toward the subject's nose. Watch the subject's eyes. Repeat it several times.
What two changes are observed in her eyes?
1)2)
What other change, not seen, must also be taking place?
These three changes are accommodation reflexes. What is the purpose of each of these reflexes?
1)2)
3)
Outline the reflex pathway involved in each of them by drawing a diagram.
(MAKE IT CLEAR AND EASY TO UNDERSTAND AND MAKE SURE THAT YOU NAME THE STIMULUS, THE RECEPTORS, THE AFFERENT AND EFFERENT PATHWAYS, THE CNS (brain or spinal cord?), THE EFFECTORS AND THE RESPONSES)
3. If your subject wears glasses, remove them for this test. Test one eye at a time, covering the other eye (ask your subject to cover her eye with her hand. Be careful not to press on the eyeball). Using the string and clothespin as described in 1 and 2 (above), Determine the minimum distance at which the clothespin is in sharp focus (slowly bring the clothespin toward the eye of your subject until she sees the clothespin becoming fuzzy. Move the clothespin away until the subject sees it again perfectly clearly and stop.). Measure the distance from your subject's eye to the clothespin: this is the near point of accommodation. Repeat twice more, average the 3 results and record your results in table 2. Determine the near point for the other eye and record it in table 2. Repeat this test for the 2 other members in the group.
Table 2: Near point of accommodation in cm.
STUDENT'S NAME / LEFT EYE / RIGHT EYEWhy is it not possible to focus clearly on an object closer than the near point?
4. Normal values for the near point change with age as follows:
Table 3: Correlation of age and near point of accommodation.
age in years / 10 / 20 / 30 / 40 / 50 / 60 / 70near point, cm / 7.4 / 8.9 / 11.4 / 17 / 52.3 / 83.3 / l00
Why does the near point increase with age?
What is presbyopia?
5. What is myopia (= nearsightedness)? What are its causes? How is it corrected?
6. What is hyperopia (= hypermetropia; = farsightedness)? What are its causes? How is it corrected?
NAME AND AGEOF STUDENT / OBSERVATIONS (same, higher or lower than value in your age group?) / CONCLUSIONS (are your eyes in the normal range? if not why?)
right eye
left eye
right eye
left eye
right eye
left eye
7. In table 4, compare your near points with normal values. Consider each eye separately. Explain any discrepancy, if possible.
Table 4: Interpretation of your results.
- BINOCULAR VISION AND DEPTH PERCEPTION.
(Seeley p. 522-523)
The eyes of many animals (rabbits, pigeons and others) are on the side of their head. Such animals see in two different directions. The crossover of the optic nerve fibers at the optic chiasma is total. This means that each visual area of the cortex receives input from a single eye and thus a totally different visual field. Rabbits and pigeons have a panoramic field of view (panoramic vision).
Humans, cats, predatory birds and most primates are endowed with binocular vision. They have both of their eyes set anteriorly, looking in approximately the same direction. The visual field of both eyes overlaps to a considerable extent, and each eye sees a slightly different view of the same image (this image being on the overlapping part of the visual fields; figure 15.21, p. 523 Seeley). Half of the optic nerve fibers cross over to the other side of the brain at the optic chiasma. This means that the visual area of the right and left cortex will receive the same image coming from each eye but viewed from slightly different angles. The cortex will integrate these slight differences between images to give depth perception (= three-dimensional vision; an accurate means of judging relative distance between objects).
EXERCISE F.
To differentiate between the visual fields of the left and right eye, do the following experiment. Focus on a clearly defined object at eye level 8 or l0 feet distant. Place your index finger on the right side of your higher eyelid and gently depress the eyeball. Keep both eyes open. What do you see?